The fluorometric assay of histamine has been automated in this laboratory, thus making available a specific, sensitive (O.OOl mug) and rapid (25 analyses per hour) procedure. The availability of the automated assay makes the laboratory diagnosis of human reaginic allergy by in vitro techniques a potential reality. A combined clinical- laboratory study is proposed in which information obtained from patient history, skin tests and in vitro release of histamine from patient leukocytes will be compared. At issue is the possibility of substituting in vitro laboratory procedures for the patient's skin in the diagnosis of human reaginic allergy as well as in follow-up studies of immunotherapy. The automated procedure will also be used in furthering mechanistic studies of histamine release from human leukocytes by IgE and from rabbit leukocytes and platelets by reaginic and non-reaginic immunoglobulins.